1. Field of the Invention
The present invention relates to a multi-stage rotary vacuum pump used for high-temperature gas, and is applicable to a vacuum pump handling gas at, for instance, about 150.degree. C. to 250.degree. C.
2. Description of the Related Arts
Where high temperature gas is sucked by a vacuum pump, a method for cooling the gas by cooling water applied outside a housing of the pump or rotors of the pump, a method for cooling the gas by a gas cooler located immediately before the inlet of the pump, or the like have been used. Since the heat of the high-temperature gas inside the housing is transferred through the shaft supporting the rotor to heat the bearings to a temperature higher than an allowable temperature, cooling from outside of a vacuum pump causes the following serious problems in using the vacuum pump. Since coolant water is to be supplied to a rotating rotor, the cooling of the gas by the coolant inside the rotating rotor must overcome structural problems. Since the gas may be solidified at a temperature lower than the solidification temperature, determined by the pressure and the temperature of the gas, in the cooling of the gas immediately before entering to the vacuum pump, it is required to design and construct a vacuum pump to overcome this problem of solidification.
A vacuum pump with first, second, and third pump sections as an example of a multi-stage vacuum pump cooled peripherally from outside is shown in FIG. 13. The XIV--XIV cross-section thereof is shown in FIG. 14, and the XV--XV cross-section thereof is shown in FIG. 15. The structure of the vacuum pump of FIG. 13 is as follows. The first pump section and the second pump section are separated by the wall 1, the second pump section and the third pump section are separated by the wall 2 as shown in FIG. 13. The first shaft 3 and the second shaft 4 penetrate each pump section, are supported by two bearing mechanisms 5, and are arranged to rotate in opposite directions by the timing gear set 6. The passage for the flow of the coolant water 7 is formed in the periphery of the housing. The first shaft, which penetrates to the outside, can be driven by an electric motor.
The suction gas G51 of the vacuum pump is sucked as a suction gas through the suction inlet 8 of the vacuum pump and the suction inlet 9 of the first section, and is transferred by the action of the rotors 10A and 10B. The exhaust gas G52 cooled by the coolant water in the periphery of the housing is led to the next stage through the communication path 11. These operations are repeated in each of the stages, and the gas is finally exhausted from the exhaust outlet 12 of the vacuum pump.
The cover 60 for the gear side is shown in FIG. 14. The input and output of the cooling water are indicated by symbol C in FIG. 15. the journal joint 13, the rotors 10A and 10B, the first axis 15, the second axis 16, the discharge openings 17 and 18 of the cooling water, and the flow C of the cooling water are shown in FIG. 16.
In this vacuum pump, the gas which is transferred and compressed by rotors in the housing is heated due to the heat of compression. A part of the heat is removed by the cooling by the coolant water 7 in the periphery of the housing. However, since the rotor is not cooled, the heat of the high temperature gas in the housing is transmitted through the shafts 3 and 4 which support the rotors to cause a problem in that the bearings are heated to a temperature higher than the allowable temperature.
In a method to solve this problem a gas cooling device is arranged immediately before the suction inlet to supply cooled gas through the suction inlet. The gas, however, is often solidified when the temperature of the gas becomes lower than the solidification temperature (which is determined by the pressure and temperature of the gas).